A printer and a projection television set require a light source for three colors of red (R), green (G), and blue (B). As the light source, there has been developed a wavelength conversion laser in which laser light of 900 nm band, 1 μm band, or 1.3 μm band is used as fundamental laser light and the fundamental laser light is wavelength-converted into harmonic laser light, for example, second harmonic generation (SHG) light or third harmonic generation (THG) light by a nonlinear material. In order to realize high conversion efficiency from the fundamental laser light to the harmonic laser light in the case of harmonic, the fundamental laser light on the nonlinear material is required to high power density and also high-intensity laser light having less wavefront aberration is required.
In a two-dimensional waveguide type laser, the power density of the fundamental laser light may be increased, and hence the high conversion efficiency to the harmonic laser light may be realized. In contrast, the increase in power is limited because of a breakdown limit accompanying the high-power density. LD light, which may be coupled to a two-dimensional waveguide and has excellent beam quality in a two-dimensional direction, is also limited, and hence the increase in power is limited.
In order to realize the wavelength conversion laser device as described above, a plane waveguide type laser having a one-dimensional waveguide may be used (see, for example, Patent Document 1). In the plane waveguide type laser, laser oscillation is performed within a plate surface in a direction vertical to a laser light axis in a spatial mode, and a beam diameter of laser light is increased in the direction or a multi-beam is formed, to thereby achieve high power. In such a laser, the LD light which is an excitation source only needs to be coupled to a plane waveguide in a one-dimensional direction, and hence a high-power broad area LD may be used and thus high-power laser light may be obtained. A multi-emitter LD in which light emitting points for LD light are arranged in the one-dimensional direction may be used, and hence higher-laser power may be obtained.
In the one-dimensional plane waveguide type laser, the laser light is confined in the waveguide in the waveguide direction, and hence a stable operation may be ensured. In contrast, the laser light spatially propagates in the plane direction, and hence it is necessary to provide a curvature mirror or a lens in a laser resonator in order to ensure the stable operation.
FIG. 13 illustrates a conventional structure. In FIG. 13, reference numeral 1 denotes a semiconductor laser, 2 denotes a comb heat sink, 3 denotes a bonding material, 4 denotes a cladding (low-refractive index portion) bonded to an upper surface of the heat sink 2 by the bonding material 3, 5 denotes a laser medium provided on an upper surface of the cladding 4, 5a and 5b denote end surfaces of the laser medium 5, and 6 denotes an optical axis indicating a laser oscillation direction of the semiconductor laser 1. In the structure illustrated in FIG. 13, excitation light from the semiconductor laser 1 enters the laser medium 5 via the end surface 5a of the laser medium 5 and is absorbed in the laser medium 5 to produce a gain for fundamental laser light in the laser medium 5. The fundamental laser light is emitted from the end surface 5b perpendicular to the optical axis 6 of the laser medium 5 at the gain produced in the laser medium 5.
In this case, in the structure illustrated in FIG. 13, heat generated in the laser medium 5 is discharged through the comb heat sink 2. In such a structure, a temperature distribution as illustrated in FIG. 14(a) is caused in the laser medium 5, and the flow of heat as illustrated in FIG. 14(b) occurs. When the laser medium 5 is excited by the semiconductor laser 1, heat generates depending on a wavelength difference between a wavelength of the excitation light from the semiconductor laser 1 and a wavelength of the light generated in the laser medium 5. Then, a temperature of the laser medium 5 increases because of excitation. A refractive index of the laser medium 5 becomes higher as the temperature thereof increases, and hence a lens effect is produced. Such a lens is called a thermal lens. The thermal lens is produced during the laser operation, and hence a stable laser operation may be performed without the need to provide an optical part, for example, a curvature mirror or a lens in the plane direction. Thus, both ends of the laser resonator may be provided to have a flat surface, and hence there is an advantage that laser power is obtained by a simple structure.
In the laser in which the resonator is formed by the thermal lens as described above, the power of the thermal lens (lens power) is required to be set in a range in which the laser resonator operates stably. In any of the cases where the power of a used thermal lens is smaller and larger than the power of a thermal lens in which a stable laser operation condition is obtained, the laser is not oscillated. In contrast, when the thermal lens is used in the range in which the stable operation condition is obtained, laser output is obtained. Even the thermal lens in the range of the stable operation condition has a feature that, when the laser is in a condition close to a non-oscillation condition, a laser resonance loss caused in the laser resonator is so large, that the laser power is low. Therefore, in order to obtain stable laser power, the thermal lens is required to have lens power in a predetermined range which is determined by the laser resonator. Thus, when the amount of heat generated in the laser medium 5 and the shape and heat capacity of the comb heat sink 2 are suitably set, the lens power of the thermal lens may be set in the stable operation range. In such a case, when the thermal lens is in a stable steady state, the high-power laser output may be stably obtained.
However, a transient response of the thermal lens occurs at the rising of pulse operation or continuous-wave (CW) operation. Heat is not generated before the operation of the laser, and hence there is no thermal lens. When heat starts to generate by excitation by the semiconductor laser, the lens power of the thermal lens increases with a time constant determined based on the amount of heat and the shape and heat capacity of the comb heat sink 2. After a lapse of a predetermined time, the lens power becomes a steady state. In this case, the lens power of the thermal lens is 0 at the instant when the excitation starts, and gradually increases with time. Therefore, the laser is in the non-oscillation state during a period from the instant when the excitation starts to the formation of the thermal lens in which the stable laser operation condition is obtained. In addition, when the lens power becomes larger, the output increases. In the case of the lens power which is in the steady state, the loss of the laser resonator is small, and hence high-power laser light is stably obtained.
Patent Document 1: WO 2005/033791 A1